Under polishing and over polishing are issues often seen in CMP processes. Under and over polishing can occur in center or edge dies and in both static random access memories (SRAMs) and in dummy structures. FIG. 1A illustrates a substrate 101 after a high aspect ratio process oxide (HARP) 103 is deposited in a trench for a STI region and over pad nitride 105 and pad oxide 107. Post STI CMP or hydrogen fluoride (HF) deglaze, in an ideal case, no oxide remains over the pad nitride 105, as illustrated in FIG. 1B. Then, pad nitride 105 may be removed by wet nitride removal, resulting in the structure shown in FIG. 1C. However, adverting to FIG. 1D, when under polishing occurs, a thin film of oxide 103′ remains on top of pad nitride 105. Consequently, during the wet nitride removal process the nitride is shielded from the etchants and cannot be removed. The wafer is therefore defective, resulting in yield loss.
Due to the unavailability of suitable metrology, under polishing is not detectable until the wet nitride removal defect scan post nitride removal, at which point rework of the problematic wafers becomes impossible. Known methodology for monitoring remaining oxide involves using an optical ellipsometer to measure film thickness on top of a solid pad based on interference, and inferring the presence of residual oxide in the cell area. However, it has been found that there is no strong correlation between the presence of residual oxide in the cell area and the relevant thickness on the measurement solid pad, since the cell area is located in a dense area whereas the solid pad is an isolated pad. When using HARP for STI, the conformal deposition yields a particularly thick layer over dense areas, resulting in even less of a correlation between the presence of residual oxide in the cell area and the thickness on the measurement pad.
In addition, the result of the pad structure post CMP measurement could also be affected by up stream processes. For example, reactive ion etching (RIE) of the STI trench, the HARP deposition and the resulting thicknesses in the trench, above the trench, and over the pad nitride, the pad nitride thickness, etc., all could affect CMP performance assessment. Further, there is only one solid pad proxy for such a measurement, with a maximum measurable area having a radius of 140 millimeters (mm), with edge areas omitted. Therefore, it is impossible to obtain a full picture of the wafer condition, especially when the field is large.
Phase modulation has traditionally been employed for topography measurement. However, phase modulation is only applicable for micron level pattern measurements and, therefore, cannot report oxide residue thickness.
A need therefore exists for methodology enabling direct measurement of residual oxide on dense structures, with no limitation on pattern size, and feedback for adjusting polishing time.